TUI: add nested store modal to the profile form

[rmanibus]

Summary

• let the profile form create a store inline instead of requiring an existing store
• add a nested store modal and return to the profile form with the new store selected
• allow editing the currently selected store from the profile modal

Scope

• type-driven store modal for local, s3, b2, and sftp URIs
• profile modal store selector now includes + Create store
• preserve existing extra store settings on edit while updating the URI

Testing

• env GOCACHE=/tmp/cloudstic-gocache go test -count=1 ./cmd/cloudstic ./internal/app ./internal/tui
• env GOCACHE=/tmp/cloudstic-gocache GOLANGCI_LINT_CACHE=/tmp/cloudstic-golangci-lint golangci-lint run ./cmd/cloudstic ./internal/app ./internal/tui

Related: #232